The present invention relates to the in situ measurement of polarization mode dispersion (PMD) in optical fibers employed, for example, in communication systems.
Recent improvements in fiber materials and transmitters have significantly reduced pulse dispersion, bringing optical fiber communications to a new bandwidth barrier imposed by polarization mode dispersion (PMD). Previously, PMD was insignificant in magnitude relative to other dispersive effects, but now has become a limiting factor for communications networks operating at bit rates of about 10 Gb/s or more. PMD which arises from the fiber""s birefringence causes the two orthogonal polarization modes of the optical signal in the fiber to travel at slightly different propagation speeds. The difference in the propagation time between the two polarization modes causes the optical signal to broaden on the order of 10-20 picoseconds (ps) in a 100 Km fiber, limiting the data rate of the network. In as much as PMD can change as a function of time due to changes in environmental conditions, a method is needed for measuring the PMD to ensure the integrity of the optical network.
In the prior art, a so-called xe2x80x9cJones-matrix eigneanalysisxe2x80x9d method has been recently proposed for measuring the PMD. See, for example, U.S. Pat. No. 5,227,623 (hereinafter the xe2x80x9c""623 patentxe2x80x9d), which is incorporated herein by reference. In the disclosed method of the ""623 patent, the transmission properties of optical devices, including optical fibers, are measured in the form of a so-called xe2x80x9cJones matrixxe2x80x9d by recording the response of the optical device for each of three known states of polarization. In doing so, the Stokes parameters of the beam transmitted through the optical device are first measured by splitting the transmitted beam into four beams, and then by passing three of them through optical anlayzers, such as polarizers. For optical fibers, this method unfortunately would require access to the end of the fiber. Otherwise, reflection measurements ostensibly must be employed which would require, among other things, that the state of polarization of the four beams incident on the optical analyzers remain unchanged by the return propagation. Furthermore, since the transmitted beam is split several times, the associated optical loss could limit the method""s useful range of applicability, particularly for long-haul optical communication networks.
Accordingly, a method and system are needed for measuring the PMD of optical fibers. Moreover, such a method should desirably be performed in situ, that is, while there is still live traffic.
The present invention employs polarizers and delay elements to effect the real-time measurement of optical parameters required to compute the polarization mode dispersion (PMD) . The measurement is performed in situ and based on the remote sensing of the intensity levels of optical pulses transmitted through two polarizers deployed along the fiber for different known states of polarization, at each of two wavelengths. In as much as information about the output states of polarization of the optical pulses contained in these latter transmission measurements are made substantially coincident with the location of the polarizers, the return propagation of the optical pulses does not affect the measured polarization characteristics of the fiber.
In a preferred embodiment, the measurement system comprises a narrowband tunable optical source which launches short duration optical pulse(s) adjusted to have six different known polarization states, at each of the two different wavelengths. The polarization characteristics of the optical fiber are determined by remotely sensing the transmission of the launched optical pulses through the two polarizers. To effect this remote sensing, a small portion of each launched optical pulse is tapped off the optical fiber and split into two xe2x80x9coptical sensing pulses.xe2x80x9d One optical sensing pulse passes along one branch through a linear polarizer, and the other passes through a circular polarizer. These optical sensing pulses are redirected back along the optical fiber, thereby allowing transmission measurements to be performed, with each pulse allocated a unique time slot using fiber-based delay lines.
With the input and output states of polarization of the optical pulses expressed as, the polarization characteristics of the optical fiber are determined in the form of a pair of Mueller matrices for each of the two different wavelengths from the intensity levels of the optical pulses transmitted through the two polarizers. With the Mueller matrices converted to corresponding Jones matrices, a Jones matrix calculus is then performed to determine the PMD.